The invention pertains generally to closed loop fuel management systems and is more particularly directed to an integral control system that responds to particular engine operating conditions by switching to an open loop operating mode while maintaining a gross system correction provided from a closed loop mode.
Conventional closed loop fuel management systems found on motor vehicles today utilize a bilevel oxygen gas sensor responding to the constituent presence or absence of oxygen in the exhaust gas of the engine. These closed loop fuel management systems generally include an integral controller which increases and decreases the air/fuel ratio above and below the stoichiometric value according to the bilevel sensor signal. A characteristic limit cycle oscillation is generated which centers the air/fuel ratio average near the stoichiometric value.
It is known that these integral control systems provide a closed loop correction or compensation for the open loop portion of an electronic control unit which responds to measured engine operating conditions such as mass air flow. The open loop portion is used to calculate a quantity of fuel to be input to the engine from the measured parameters and schedules a predetermined air/fuel ratio, usually stoichiometric therefrom. For emission control a three-way catalytic converter with a narrow air/fuel ratio conversion window is commonly included in such a system. The closed loop control adaptively interacts with the open loop portion and retains the scheduled air/fuel ratio within the well defined narrow air/fuel ratio limit for the efficient reduction of exhaust emissions by the catalytic converter.
An advantageous example of such an integral controller is shown in U.S. Pat. No. 3,990,411 issued Nov. 9, 1976 to Oberstadt et al., which is commonly assigned with the present invention and the disclosures of which is hereby incorporated by reference herein.
In Oberstadt et al., a cascaded controller is disclosed where primary and secondary integrators are used in a closed loop control mode. The primary integrator, which has a relatively fast integration or ramp rate, is utilized for transient control and rapidly follows an indication of a need to correct the air/fuel ratio. Such a fast integration rate, however, with the inherent system lags in the integral control law will produce large air/fuel ratio excursions if the primary integrator is allowed to have a significant authority. Such large excursions would exceed the bounds of the narrow band of air/fuel ratios necessary for efficient catalytic conversion.
Thus, a secondary integrator with a relatively slow integration or ramp rate is used for gross control and has a much larger authority level than the primary integrator. The secondary integrator is used primarily for compensating the ageing effects of the engine and for altitude compensation which produce slowly varying but large or gross changes in the need for air/fuel ratio correction of the open loop calibration. The secondary integrator can be envisioned as providing a gross operational offset around which the primary integrator can limit cycle.
One useful feature of the Oberstadt et al. system is its ability to switch from closed loop to open loop control while detecting certain special engine conditions and rich fuel power demands. Generally, the more important of these conditions are at idle, wide open throttle, and when the engine operating temperature is cold. During these periods, the engine generally will require a richer air/fuel ratio than the stoichiometric value that the closed loop mode provides and the system is switched to an open loop mode to output this value. Normally, this switching from closed loop to open loop control provides an advantageous system whereby the system operates most of the time in the narrow band around stoichiometric, and only when the particular special engine operating conditions are detected does it generate a richer air/fuel ratio. The primary and secondary integrator are clamped to noncorrectional values during this open loop mode of operation.
It is now recognized, however, that the secondary integrator which provides the gross operational control of the system is necessary for correction of the open loop air/fuel ratio even during those special engine operating conditions mentioned above. When operating at the predetermined richer air/fuel ratio generated for these conditions, ageing factor and altitude compensation corrections are needed just as much as they are needed when the system is acting under closed loop control. The information necessary for developing these corrections is stored as the instantaneous operating point of the secondary integrator. The conditions which cause the voltage level to vary on the secondary integrator may build over long time periods and can cause significant air/fuel ratio errors when running open loop if the correction is not utilized. However, in the present Oberstadt et al system this information is lost when the system switches into an open loop mode of operation by clamping the integrators and must be regenerated upon the return to closed loop control.
Further, because the secondary integrator has a slower integration rate and a much greater authority than the primary integrator, once the special conditions cease and switching back to closed loop control occurs, the Oberstadt et al. system responds relatively slowly in regaining the operational point of the secondary integrator. The greater the original gross correction, the longer it will take for the system to regenerate the correction and the greater the air/fuel ratio error will be during the delay.
Therefore, it would be highly desirable to maintain open loop system control during special engine operating conditions while providing the secondary integrator operating point as a correction of value to the open loop control. As a consequence a faster and more facile switching back into the closed loop mode would also be obtained by such control because the gross correction would not have to be regenerated.